T. GooDEY 43 



They must, within quite a short period after ingestion, become 

 accustomed to their new temperature and surroundings and resume 

 fairly active motility in order to escape from their sheaths. It is possible 

 that they require the specific stimulus of contact with their final en- 

 vironment, the stomach and intestinal walls respectively for G. strigosmn 

 and T. retortaefornm, for their emergence and further growth, though 

 Veglia((y) p. 426) has shown that the larvae of Haemonchiis conlorlus, 

 when taken by lambs with grass, can escape whilst in the mouth. 



As bearing on this point I would refer to the observation recorded 

 above, p. 40, in which I mention having found the larvae of G. strigosum 

 and T. retortaefonnis active after being in contact with the skin for 

 nearly 48 hours at 37° C. 



RESISTANCE TO DESICCATION AND EFFECT OF 

 PLASMOLYSING SOLUTIONS. 



Intimately associated with the power possessed by the larvae of 

 Necator and Ancijlostoma to penetrate skin is, I think, their lack of power 

 to withstand desiccation. Looss ((4) p. 398) deals with the latter at length 

 and says that " Ancylostoma larvae can remain alive on a surface which 

 is becoming dry (and to which they adhere) so long as the envelopes 

 surrounding them (the 'cysts' or 'sheaths') retain moisture within 

 them. As soon, however, as this moisture begins to evaporate the bodies 

 contract (generally in a longitudinal direction) and shrivel." 



It is a very simple matter to allow a drop containing ensheathed 

 larvae of N. americanus to evaporate gradually from a slide and in the 

 course of my work I have performed the experiment a few times. It is 

 difficult after the bulk of the water has dried up to make out the structure 

 of the larvae, but it is very evident that they quickly feel the effects of 

 desiccation even at room temperature, for if a drop of water is added to 

 the slide after a few minutes, they fail to revive and resume active 

 motion. None revive if the slide is allowed to remain dry for 15 minutes. 

 It seems a natural inference to draw therefore that Necator and 

 Ancylostoma larvae seek the protection afforded by penetration 

 into skin because if they remained outside and became dry they would 

 perish. 



The ensheathed larvae of G. strigosum and T. retortaeformis on the 

 other hand can withstand desiccation in the air for a time and when 

 remoistened can revive and resume normal activity. Prolonged desicca- 

 tion at high temperature and the action of direct sunlight are inimical 

 to them as Veglia ((9) pp. 390 et seq.) has shown in the case of Haemonchus 



